Short stories, especially flash fiction, are self-contained (beginning/middle/end). Sharp, hard-hitting, unforgettable. A story you can read in ten minutes but remember for a lifetime. Or at least a month or two... :) No back story, time shifts or passive voice. Readers see and hear everything through your main character's eyes, ears and thoughts.
Title => hook readers with an attention grabbing title that says "Come on in. This story is worth your time."
Beginning => start with a compelling character in conflict with a unique, attention getting problem.
Middle => deepen the conflict by showing how that external problem relates to his/her internal struggles.
End => wrap your story up quickly and completely so readers are not left with a confused or to-be-continued feeling. Finish in a way that resonates with the beginning so your readers see the deeper significance of the story.
Make every word count so every sentence reveals more about your character and moves the story forward
Focus on the quality of the words you use (language) => rhythm, subtlety, sensory detail.
Focus on the quality of the story (plot) => Brevity (word count) is about quantity, whereas concision (keep it simple stupid) is about quality (maximize showing, minimize telling, sharp images, nouns that don't need adjectives, active verbs that don't need adverbs).